The accumulation
buffer is an extended-range color buffer. Images are not rendered into it.
Rather, images rendered into one of the color buffers are added to the
contents of the accumulation buffer after rendering. Effects such as antialiasing
(of points, lines, and polygons), motion blur, and depth of field can be
created by accumulating images generated with different transformation
matrices.

Each pixel in the accumulation buffer consists of red, green,
blue, and alpha values. The number of bits per component in the accumulation
buffer depends on the implementation. You can examine this number by calling
glGetIntegerv four times, with arguments GL_ACCUM_RED_BITS, GL_ACCUM_GREEN_BITS,
GL_ACCUM_BLUE_BITS, and GL_ACCUM_ALPHA_BITS. Regardless of the number of
bits per component, the range of values stored by each component is [-1, 1].
The accumulation buffer pixels are mapped one-to-one with frame buffer pixels.

glAccum operates on the accumulation buffer. The first argument, op, is
a symbolic constant that selects an accumulation buffer operation. The second
argument, value, is a floating-point value to be used in that operation.
Five operations are specified: GL_ACCUM, GL_LOAD, GL_ADD, GL_MULT, and
GL_RETURN.

All accumulation buffer operations are limited to the area of
the current scissor box and applied identically to the red, green, blue,
and alpha components of each pixel. If a glAccum operation results in a
value outside the range [-1, 1], the contents of an accumulation buffer
pixel component are undefined.

The operations are as follows:

GL_ACCUM

Obtains
R, G, B, and A values from the buffer currently selected for reading (see
glReadBuffer). Each component value is divided by $2 sup n^-^1$, where $n$
is the number of bits allocated to each color component in the currently
selected buffer. The result is a floating-point value in the range [0, 1],
which is multiplied by value and added to the corresponding pixel component
in the accumulation buffer, thereby updating the accumulation buffer.

GL_LOAD

Similar to GL_ACCUM, except that the current value in the accumulation
buffer is not used in the calculation of the new value. That is, the R,
G, B, and A values from the currently selected buffer are divided by $2
sup n^-^1$, multiplied by value, and then stored in the corresponding accumulation
buffer cell, overwriting the current value.

GL_ADD

Adds value to each R,
G, B, and A in the accumulation buffer.

GL_MULT

Multiplies each R, G,
B, and A in the accumulation buffer by value and returns the scaled component
to its corresponding accumulation buffer location.

GL_RETURN

Transfers
accumulation buffer values to the color buffer or buffers currently selected
for writing. Each R, G, B, and A component is multiplied by value, then
multiplied by $2 sup n^-^1$, clamped to the range [0,$~2 sup n^-^1 $], and stored
in the corresponding display buffer cell. The only fragment operations that
are applied to this transfer are pixel ownership, scissor, dithering, and
color writemasks.

To clear the accumulation buffer, call glClearAccum with
R, G, B, and A values to set it to, then call glClear with the accumulation
buffer enabled.